Conventionally, various kinds of what is called flash apparatus capable of varying the emission angle have been put to practical use or proposed which are configured to control the emission angle of light emitted from a light emitting portion comprising a flash discharge tube as a light source, a reflector, a Fresnel lens as an optical member, and the like according to the focal length of a photographing lens used for taking a photograph.
FIG. 10 is a partial schematic view of a conventionally well-known flash apparatus capable of varying the emission angle. The flash apparatus is capable of varying the emission angle by moving a Fresnel lens 4 disposed in front of a light emitting unit 1 comprising a reflector 2 and a flash discharge tube 3 disposed at the bottom of the reflector 2 as a light source from a position X shown with solid lines to a position Y shown with broken lines. It is to be noted that the opposite, that is, an apparatus configured to have the fixed Fresnel lens 4 and the movable light emitting unit 1 is also well-known (such as ones disclosed in Japanese Utility Model Application Laid-Open No. 53-62330 and Japanese Patent Application Laid-Open No. 60-83921).
FIGS. 11A and 11B are partial schematic views of another conventionally well-known flash apparatus capable of varying the emission angle. The flash apparatus is capable of varying the emission angle by changing the positional relationship of the flash discharge tube 3 with respect to the reflector 2 by way of moving the flash discharge tube 3 from a position X1 shown in FIG. 11A on the "wide" side to a position Y1 shown in FIG. 11B on the "telescopic" side (such as one disclosed in Japanese Patent Publication No. 62-51453).
However, conventional flash apparatus capable of varying the emission angle described in the above has the following disadvantages.
For example, with respect to the apparatus shown in FIG. 10 capable of varying the emission angle basically by changing the relative positional relationship between the light emitting unit 1 and the Fresnel lens 4, it is known that the amount of the relative movement of the two necessary to obtain the required change in the emission angle is relatively large compared with the amount of the movement of the reflector 2 or of the flash discharge tube 3 of the apparatus shown in FIGS. 11A and 11B capable of varying the emission angle by changing the positional relationship of the flash discharge tube 3 with respect to the reflector 2. Therefore, the apparatus basically has a disadvantage of becoming larger.
On the other hand, the one shown in FIGS. 11A and 11B is advantageous in that the apparatus can be miniaturized since the amount of movement of the flash discharge tube 3 necessary to obtain the required change in the emission angle can be made smaller than the amount of movement of the light emitting unit 1 and the like of the one shown in FIG. 10.
However, the light emitting portion typically comprises a combination of a bar-like light source as the flash discharge tube 3 and the Fresnel lens 4 as an optical member.
The Fresnel lens 4 is, as is known well, configured to be like a flat plate as a whole by dividing a curved face of a lens into concentric curved face portions and by connecting the divided curved face portions together at respective sections. The Fresnel lens 4 is manufactured by injection molding or press molding of a plastic material such as acrylic resin.
Therefore, when the light emitting portion is miniaturized and actually used in taking a photograph, thin lines of various colors (hereinafter referred to as rainbow-like unevenness) are generated at the longitudinal ends of a bar-like light source L as unevenness of light distribution.
FIG. 12 is a view illustrating a state of light incoming from a bar-like light source to a Fresnel lens and passing through the Fresnel lens and the like.
For example, a photograph was actually taken and developing and printing thereof were carried out with a specific example of the figure where a longitudinal width D of a reflector K is 35 mm, a distance D1 between the center of the bar-like light source L and the bottom of the reflector K is 6 mm, a distance D2 from the center of the bar-like light source L and a plane of incident light of a Fresnel lens F is 11 mm, a distance D3 between main electrodes of the bar-like light source L is 28 mm, and the focal length of the Fresnel lens F is 35 mm. The resultant photograph was observed to have rainbow-like unevenness in Z directions which are at about 30 degrees from the center of the Fresnel lens F to both longitudinal ends of the bar-like light source L with the direction right ahead of the Fresnel lens F being 0 degree.
The above-mentioned rainbow-like unevenness is assumed to occur because, since miniaturization of a light emitting portion as a combination of the bar-like light source L and the Fresnel lens F involves smaller distance between the bar-like light source L and the Fresnel lens F, for example, angle of incidence Q1 of light shown as L1 in FIG. 12 emitted from a right end portion Lb of the bar-like light source L and incoming to a left end region Fa of the Fresnel lens F is larger than angle of incidence Q2 of light L2 emitted from the right end portion Lb and incoming to the left end region Fa in case the distance D2 between the bar-like light source L and the Fresnel lens F is large and the rainbow-like unevenness is not remarkable as shown with broken lines.
Such a situation also occurs with respect to light emitted from a left end portion La of the bar-like light source L and incoming to a right end region Fb of the Fresnel lens F and light reflected by the reflector K around the left or right end portions La or Lb of the bar-like light source L and incoming to the left or right end regions Fa or Fb of the Fresnel lens F, though not shown in the figure.
In other words, the Fresnel lens F is assumed to influence as a prism more strongly incident light from around the left and right end portions La and Lb of the bar-like light source L having larger angle of incidence. Thus, for example, the light L1 incoming to the Fresnel lens F with the angle of incidence Q1 is assumed to be emitted from the Fresnel lens F as light L3 dispersed with angle of emergence being about 30 degrees to both longitudinal ends of the bar-like light source L after being influenced by refractive action including spectral action by the Fresnel lens F. Similarly, the light reflected by the reflector K around the left or right end portions La or Lb of the bar-like light source L is assumed to be emitted from the Fresnel lens F after being dispersed by the Fresnel lens F.
Therefore, the rainbow-like unevenness appearing on a photograph when the photograph is actually taken is assumed to be generated by dispersed light such as the emitted light L3 as mentioned in the above.
By the way, let us take again the respective prior art mentioned in the above. In the respective prior art, though members to be moved to make the apparatus capable of varying the emission angle differ to some extent, the relative position of the bar-like light source and the Fresnel lens is changed between what is called a state on the wide side corresponding to a wide angle photographing lens and what is called a state on the telescopic side corresponding to a telescopic photographing lens. More specifically, in the state on the wide side, the flash discharge tube 3 as the bar-like light source and the Fresnel lens 4 are close to each other, and thus, each of the respective prior art has a disadvantage that, when the light emitting portion is miniaturized to miniaturize the apparatus, rainbow-like unevenness occurs especially in the state on the wide side.